1. Field of the Invention
This invention concerns a method of manufacturing a distributed feedback type semiconductor laser, which is most suitable to the use of manufacturing a distributed feedback type semiconductor laser with an intense coupling between light and grating.
2. Description of the Prior Art
A distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) has been viewed as a laser capable of realizing a single longitudinal mode oscillation.
In the DFB laser, primary or secondary grating has been used thus far in which undulations are formed uniformly as a grating (diffraction lattice) for distributingly feeding back light in accordance with Bragg's reflection.
Referring to the primary grating of such devices, although an effective distributed feedback can be obtained due to intense coupling with the light, the device cannot be manufactured easily or with desired reproducibility.
Further, in the secondary grating in which the primary Fourier's component is 0 due to the shape, as shown in FIG. 3, since the amount of coupling with the light is small as an absolute value as compared with that of the primary grating and, in addition, the intensity of the coupling is sensitively dependent on the shape of the grating, the coupling, may be reduced to 0 depending on the case. Accordingly, it is difficult to obtain a stable oscillation with a sufficient strength in a DFB laser using the secondary grating as shown in FIG. 3.
However, it has been confirmed theoretically and experimentally that a sufficient coupling can be obtained as the DFB laser (coupling coefficient of greater than 100 cm.sup.-1) by using the secondary grating of a shape as shown in FIG. 4, in which the primary Fourier's component is maximized. Accordingly, it is desired to provide a technique for manufacturing a secondary grating of the shape as shown in FIG. 4 uniformly and at a good reproducibility.
However, in the holographic exposure method which is a typical means for manufacturing a grating at a pitch A of about from 0.1 to 0.4 .mu.m, it is almost impossible to manufacture the grating as shown in FIG. 4 because of the distribution in the exposure intensity inherent in to the means. On the other hand, it is possible to manufacture the shape as shown in FIG. 3 in a self-aligned manner at a good reproducibility so long as the distribution of the exposure intensity is within a certain extent, because of the anisotropy (crystal face dependency) possesses by a semiconductor substrate and an etching solution.
Further, since an electron beam exposure method can be employed if A&lt;0.4 .mu.m, it is possible to manufacture a secondary grating of the shape as shown in FIG. 4 with good reproducibility. However, it is impossible to use such a method for the DFB laser with the wave length at the vicinity of 850 nm since the pitch for the secondary grating is of about 0.25 .mu.m.
The object of this invention is to provide an improved method of manufacturing a distributed feedback type semiconductor laser without the above noted drawbacks in the prior art.